This laboratory has developed a sensitive bioassay for serum thymic hormone. This has established applications in clinical studies on the thymus. One study has been on patients with myasthenia gravis. This study showed that abnormal levels of serum thymic hormone is not a prerequisite for severe myasthenia. A sustained lowering of thymic hormone output may be a key factor in clinical benefit from thymectomy. 1. Serum thymic hormone activity may gradually rise after thymectomy as is often the case with incomplete removal of other endocrine organs. We wish to document this gradual recovery of thymic hormone secretion by ectopic thymic tissue after thymectomy. (2) We wish to measure seum thymic hormone on patients at least three years after thymectomy for myasthenia. Groups of patients who did and did not benefit from surgery will be tested. The aim is to establish that clinical benefit correlates with persistent lowering of serum thymic hormone as was suggested by our preliminary studies. (3) 10 per cent of myasthenic patients have thymoma. We have serum from a number of patients with thymoma. We want to see if serum thymic hormone is altered with a particular histologic type of tumor or with neoplasia. (4) Through number 2 we will identify patients over three years after thymectomy with negligible serum thymic hormone activity. Animal studies suggest T cell functions are impaired after longterm thymic deprivation. We wish to evaluate effector, helper and suppressor T cell function on these selected patients.